The present invention relates to a novel magnet block assembly for an insertion device which is inserted into the linear part of an electron accelerator or electronic storage ring to emit a synchrotron radiation of high intensity. More particularly, the invention relates to an assembly of permanent magnet blocks for a compact-size insertion device of a small period length having a large number of periods despite the compactness as well as to a method for the magnetization of the magnet blocks in the assembly.
As is known, an insertion device is a device inserted into the linear part of an electron accelerator or electronic storage ring to emit a synchrotron radiation of high intensity. An insertion device of the prior art is a device, as is illustrated in FIG. 3A by a perspective view, having a structure of a magnet block assembly consisting of at least two arrays of permanent magnet blocks disposed to oppose each the other to form an air gap therebetween. When the directions of magnetization of the individual permanent magnet blocks are as shown in FIG. 3A indicated by the small arrows on the end surfaces of the respective magnet blocks, as is illustrated in FIG. 3B, a periodical magnetic field is generated in the air gap between the opposite arrays of the magnet blocks as indicated by the sine curve within the plane defined by the axes Z and Y in FIG. 3A. The insertion device to generate such a periodical magnetic field are classified into two types including, one, those of the Halbach type composed of permanent magnet blocks 20, 30, 40, 50, . . . only as is schematically illustrated in FIG. 4A by a side view and, the other, those of the hybrid type of which each array is composed of alternately arranged permanent magnet blocks 30, . . . and blocks of a soft magnetic material or pole pieces 32,.
When high-speed electrons travelling in an electron accelerator enter the periodical magnetic field between the arrays of magnet blocks along the direction Z in FIG. 3A, the electron takes a meandering motion within the plane defined by the axes Z and X as is illustrated in FIG. 3C to emit a synchrotron radiation at each of the meandering points as is reported by Halbach in Nuclear Instruments and Methods, volume 187, page 109 (1981). The mode for the emission of the synchrotron radiation is called either a wiggler mode or undulator mode depending on the extent of meandering of the electrons. In the wiggler mode emission, the radiations emitted at the respective meandering points are superimposed to give a white synchrotron radiation having an overall intensity 10 to 1000 times higher than the radiation from a bending electromagnet. In the undulator mode radiation, on the other hand, the radiations emitted from the respective meandering points interfere each with the others to give a radiation intensity 10 to 1000 times still higher than the wiggler mode radiations relative to the fundamental radiation and higher harmonics thereof. The differentiation between the wiggler mode radiations and undulator mode radiations can be made in terms of the value of a parameter K=0.934 .lambda.m (m).multidot.Bg (Tesla), where .lambda.m is the length of a period and Bg is the peak value of the periodical magnetic field. Namely, an undulator mode is obtained when the value of K is about 1 or smaller while the radiation is of the wiggler mode when K is otherwise. For simplicity and convenience, the terms of undulator and insertion device are used in the present invention to cover both of these two modes. Further, in the following description, the "air gap direction" means the direction from a magnet block in a first magnet block array to a magnet block in a second magnet block array to oppose the magnet block in the first array or, namely, the direction of the axis Y in FIG. 3A. The "axial direction" in the following description means the direction of the orbit of electrons entering and traveling through the periodical magnetic field between the magnet block arrays or, namely, the direction of the axis Z in FIG. 3A.
While, as is mentioned above, insertion devices are grossly classified into those of the Halbach type and those of the hybrid type, no great differences are found therebetween relative to the value and distribution of the magnetic field. Generally speaking, however, the overall weight of the magnet blocks can be smaller in the hybrid type ones than in the Halbach type ones. In addition, the hybrid type insertion devices were preferred in the early stage of development when the manufacturing technology was at a low level not to give magnet blocks with high accuracy relative to the value and angle of magnetization in the magnet blocks while the requirements for the accuracy of the above were lower in the hybrid type than in the Halbach type. In recent years, however, a satisfactory magnetic field distribution can be obtained in each of the insertion devices of the Halbach type and hybrid type as a result of the improvement in the magnet manufacturing technology and introduction of the method for recombination of magnet block pairs. The displacement of the electron orbit caused by the change in the air gap spacing is smaller in the Halbach type than in the hybrid type due to the linearity held therein as compared with the hybrid type with non-linearity of the soft-magnetic pole pieces 32 to cause a relatively large displacement of the electron orbit. The magnet block arrays illustrated in FIGS. 4A and 4B are each conventional and called a planar undulator. Accordingly, choice of either one of these types is not a matter of superiority or inferiority but entirely depends on the particularly intended application of the insertion device.
The most conventional method for fixing and assembling permanent magnet blocks into an array is illustrated in FIG. 5 by a cross sectional view within the plane X-Y in FIG. 3A. Thus, the magnet block 20 is set in a rigid cassette 21 of a non-magnetic material and fixed at the position either by using an adhesive or by a mechanical means with presser plates 23 and screw bolts 24. The adhesive means and mechanical means can be used in combination. Basically, the mechanical means has higher reliability than adhesive bonding. The magnetic field generated by the magnet block can be adjusted by means of the adjustment hole 22 formed on the bottom or on the side wall of the cassette 21. Since the cassette 21 can be prepared by mechanical working using precision machine tools, the dimensional accuracy of the cassette 21 is generally high as compared with the magnet block 20. While the positioning accuracy of the magnet blocks 20 in the length-wise direction of the magnet block array is particularly important, the positioning accuracy of the magnet blocks as required can be obtained when the accuracy in the dimension of the cassette 21 and the screwing females for the screw bolts 23 is ensured. In view of these advantages, the permanent magnet blocks 20 in the insertion devices are usually fixed and assembled by using a cassette 21 in most cases.
The above mentioned advantages obtained by using a cassette for assembling a number of magnet blocks, however, are no longer held when the period length (see FIG. 3A) of the insertion device is small with a consequently small thickness of each of the magnet blocks. Suppose an insertion device of the Halbach type having a period length of 10 mm, in which a single period is formed from four magnet blocks, the thickness of each of the magnet blocks is only 2.5 mm. Since the orbit form of the accelerated electrons in an insertion device is greatly disturbed by the non-uniformity in the magnetic characteristics of the individual permanent magnet blocks, it is essential to minimize the errors in the remnant magnetization and angle error of magnetization When the thickness of the individual magnet blocks is very small, nevertheless, the error in the magnetic characteristics is unavoidably increased due to superimposition of several factors including (1) an increased error in the dimensions of the magnet blocks relative to the thickness, (2) a relative increase in the volume proportion of the work-degradation layers caused by the mechanical working of the magnet blocks, and (3) an increase in the error of the relative thickness of the anti-corrosion surface layer. These errors are superimposed onto the usual error in the magnetic properties as a consequence of the powder metallurgical method for the preparation of the permanent magnet blocks.
Other problems are caused also in respect of the accuracy of assembling of the magnet blocks. Since it is a usual design of insertion devices that the air gap spacing between the oppositely facing magnet blocks in two arrays is selected to be about one half of the period length, an insertion device of a period length of 10 mm is used with an air gap spacing of about 5 mm. While the dimensional error in a permanent magnet block prepared by mechanical working usually cannot be much smaller than .+-.0.05 mm, an error of .+-.12% is expected as a possible maximum in the magnetic field in the air gap direction and an integrated error of .+-.4% is expected as a possible maximum in the magnetic field in the axial direction. Accordingly, it is a requisite in an insertion device having a period length of 10 mm that the error in the dimensional accuracy of the permanent magnet blocks used therein must not exceed one half or one third of that in an insertion device having a conventional period length of 30 mm or larger.
The above mentioned high accuracy requirement in the dimensions of the individual permanent magnet blocks is of course of little significance unless being accompanied by the accuracy in assembling of the magnet blocks into an array, which can be obtained only with a difficulty. Assuming that the magnet blocks 20 of each 2.5 mm thickness are assembled each by using a non-magnetic cassette 21, as is illustrated in FIG. 5, to form a Halbach type insertion device of 10 mm period length, for example, the width of the presser plate 23 must be very small and the size of the screw bolts 24 must be correspondingly so small because the thickness of the cassette 21 is also 2.5 mm to hold a single magnet block 20. The screw bolt 24 thrusted into the female in the cassette of 2.5 mm thickness cannot be larger than the screw bolt of the Ml size in consideration of the difficulty in tapping of the female thread and the size of the bolt head. Since the magnetic attractive force between the oppositely facing two permanent magnet blocks in the two arrays is so strong that no very reliable assemblage of the magnet blocks can be ensured with so feeble holding means with tiny screw bolts 24. Although it is a seemingly possible way that the permanent magnet blocks are directly fixed to a single base plate instead of using separate cassettes, this way is not always practical because gap spaces are sometimes formed between adjacent magnet blocks due to the repulsive and rotational forces therebetween resulting in inaccuracy in the positioning of the magnet blocks in the length-wise direction of the magnet block array and consequently in an increased error in the magnetic field distribution within the air gap between the magnet block arrays.
In view of the above described problems and disadvantages in the prior art in the preparation of a permanent magnet block assembly for an insertion device having a period length not exceeding 10 mm, it is eagerly desired to develop a novel method for assemblage of thin permanent magnet blocks apart from a mere improvement or extension of the prior art methods.
One of the inventors, together with a co-inventor, previously proposed, in Japanese Patent Kokai 8-255726, a magnet block assembly for a short-period insertion device in which, as is schematically illustrated in FIG. 6, a plurality of magnet blocks are assembled in an array and magnetized with high precision in alternately reversed directions perpendicular to the length-wise direction of the array. The magnet block arrays there proposed serve to realize an insertion device of a period length not exceeding 20 mm. The characteristic advantages obtained with this magnet block assembly include a decrease in the requirement for the dimensional accuracy of the individual magnet blocks because a single permanent magnet block here covers a period or more in a conventional Halbach type insertion device composed of four or more magnet blocks, a decreased problem due to the working-degraded surface layer of the magnet blocks, applicability of the conventional assembling method with non-magnetic cassettes and a decrease in the assembling accuracy of the magnet blocks as a consequence of the decrease in the number of the magnet blocks. This method, however, has different difficulties relating to the accuracy in the distribution of the magnetic field for the magnetization of the magnet blocks and precision control of the positions of magnetization.
When magnetization of the magnet blocks is conducted consecutively with pulses of magnetic field by using a magnetization head having a coil, it is unavoidable that the electric resistance of the coil is gradually increased as the temperature thereof is increased as a result of heat generation therein to cause a shift in the distribution of the pulsed magnetic field. Since the magnetization behavior of a rare earth-based permanent magnet is non-linear relative to the magnetic field for magnetization, the magnetization pattern of the permanent magnet blocks is accordingly subject to a change thereby. This phenomenon is particularly remarkable at the boundary of the N-pole and the S-pole such as the boundary regions between the magnet block 20 and the adjacent blocks 40. As a consequence, a disturbance is caused in the distribution of magnetic field around the undulator formed by assembling the permanent magnet blocks resulting in irregularity of the electron orbit in the insertion device.
It is important in the magnetization of the magnet blocks of an undulator to exactly control the positions of magnetization. Any irregularity in the magnetization positions of the magnet blocks results in an irregular distribution of the thickness of the individual magnet units. It is necessary accordingly that positioning of the magnetization head or relative positioning of the magnetization head and the permanent magnet blocks has an accuracy with an error of .+-.0.05 mm or, desirably, .+-.0.02 mm or smaller. This very strict requirement can be satisfied only by the use of a precision-controlled driving system for the magnetization head.